Relative humidity sensor

ABSTRACT

An improved humidity sensing element of the electrical resistance-dependent, metal oxide type in which the metal oxide is sintered to achieve a stable cross linked porous metallurgical matrix. In the preferred embodiment, the humidity sensing element of the invention is produced by coating a dielectric blank to which an electrode system has been applied with a metal oxide selected from a group including the oxides of Group VIII, Period 4 of the Periodic Table, preferrably Fe2O3, applying a slight amount of compression to the coating and releasing the compression, heating the coated blank to a sufficient temperature and for a sufficient length of time to achieve sintering of the metal oxide and thereafter impregnating the cooled coating of the sensor with a reversably hygroscopic humectant material such as polyethylene glycol, for example.

United States Patent 1191 Nicholas et al.

[ Oct. 28, 1975 RELATIVE HUMIDITY SENSOR Primary Examiner-C. L. Albritton [75] Inventors: Merle E. Nicholas, Crystal; Ernest g a or Flrm charles Mersereau;

A. Satren, Cottage Grove, both of emy anson Minn.

[73] Assignee: Honeywell Inc., Minneapolis, Minn. [57] ABSTRACT 22 Filed; Jam 2 74 An improved humidity sensing element of the electrical resistance-dependent, metal oxide type in which [21] Appl- 435,614 the metal oxide is sintered to achieve a stable cross linked porous metallurgical matrix. In the preferred 52 US. Cl 338/35; ZOO/61.04 embodiment, the humidity Sensing element of the 51 1m. (:1. ..11o1c 13 00 ventieh is Produced y coating a dielectric blank to [58] Field of Search 338/34, 35; ZOO/61.04; which an eleetrede System has been pp with 8 3 5; 7 /336 5 metal oxide selected from a group including the oxides of Group VIII, Period 4 of the Periodic Table, prefer- 5 References Cited rably Fe O applying a slight amount of compression UNITED STATES PATENTS to the coating and releasing the compression, heating the coated blank to a sufficient temperature and for a g; iquler sufficient length of time to achieve sintering of the 3l05214 9/1963 fiz metal oxide and thereafter impregnating the cooled 3:295:088 12/1966 snhth .13. I: 338/35 coating of the Sensor with a reversably hygroscopic 3,686,606 8/1972 Thoma 1 1 338/35 humectant material such as polyethylene glycol, for 3,703,697 11/1972 Nicholas 338/35 p 3,715,702 2/1973 Nicholas 338/35 8 Claims, 2 Drawing Figures INERT DIELECTRIC BLANK H v 2 ELECTRODE APPLY ELECTRODES MATERAL APPLY mow 0x105 (H2 0 COMPACT 10 PSI r0 I50 PSI M N HEAT aoo c To |200c HOLD I5 Mm To 4HRS I6 COOL I7 IMPREGNATE HUMECTANT 8''\ 2| CURE I ATTACH ELECTRICAL CALIBRATE LEADS US. Patent 'Oct.28, 1975 Sheet10f2 3,916,367

INERT IOQ DIELECTRIC BLANK H ELECTRODE APPLY ELECTRODES MATER'AL APPLY IRON OXIDE (Fe O COMPACT l0 PSI TO I50 PSI '4 N HEAT 800C T0 200% HOLD I5 MIN TO 4HRS I6 COOL IMPREGNATE WITH HUMECTANT CURE ATTACH ELECTRICAL CALIBRATE LEADS FIG-l US. Patent 0m. 28, 1975 Sheet 2 of2 3,916,367

1 1 --2o Z 1 W l k L a 1 le FIG. 2

RELATIVE HUMIDITY SENSOR BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is related broadly to devices for sensing relative humidity and, in particular, to a specially treated, metal oxide, resistance type moisture sensitive element and method of fabrication.

2. Description of the Prior Art In the prior art it has been recognized that the electrical resistance of certain metal oxides varies with changes in the relative humidity of the environment to which theoxide composition is exposed. The term relative humidity" as commonly defined and used herein refers to the ratio of the quantity of water vapor actually present in the air to that amount which would saturate the air at the same temperature. It has also been recognized that relative humidity sensors could be fabricated utilizing the principle of changing electrical resistance'with changing relative humidity inherent in these metal oxide materials. Of these, the most successful relative humidity sensors have been fabricated utilizing a metal oxide selected from the group including the oxides of the metals found in Group VIII, Period 4 of the Periodic" Table.

One such device is illustrated and described in US.

Pat. No. 3,715,702issued to M. E. Nicholas, a coinventor in the present application, which is assigned to the same assignee as the present application. That invention is also directed to a resistance-type relative humidity sensor which utilizes a metal oxide coating overlapping metal electrode system spaced on an inert dielectric blank. That humidity sensor also uses a metal oxide selected from the group including oxides of the metals found in Group VIII, Period 4 of the Periodic Table and an amount of humectant material impregnated in the pores of the metal oxide film.

By that invention, the metal oxide layer is normally applied with an organic dispersent binder in an aqueous slurry. The blank is then fired to a temperature sufficient only to pyrolyze and remove the binder material. The sensor'then relies on the natural attraction of the metal oxide particles for each other and for the electrode material to maintain the porous structure and retain the coating on the blank.

While the relative humidity sensing elements produced in accordance with the above-cited Nicholas patent have been successful, there has been a need to further stabilize theporous metal oxide matrix and improve both the cohesion between the molecules comprising the metal oxide coating and adhesion between the metal oxide coating and the electrode system. This has been needed to stabilize the longterm calibration of the sensor, i.e., the relation between the electrical resistance exhibited by the sensor and the relative humidity of the atmosphere to which it is exposed. Also, im-

' proved adhesion and cohesion, of course, improves the physical stability of the sensor by eliminating flaking or chipping of the oxide coating.

SUMMARY OF THE INVENTION with improved physical stability by achieving an improved cohesion between the molecules of the metal oxide involved and improved adhesion between the molecules of the metal oxide and the electrode system of the sensor. In the preferred embodiment, a base member formed of a dielectric, substantially inert material such as quartz, for example, is supplied with a suitable electrode system such as one of sputtered platinum. The conductance path between the electrode system is provided by a sintered, highly porous metal oxide coating containing a reversably hygroscopic material. The metal oxide coating is normally applied as a powder and sintered in place (after being subjected to a slight amount of pressure to achieve some compaction) by heating to a temperature which is sufficient to achieve a true sintered matrix but one which is highly porous in nature. The sintering step has been found to achieve a superior metal oxide to metal oxide bond and a superior metal oxide to electrode bond which substantially stabilizes both electrical resistance and physical properties in the sensor. The coated blank is then impregnated with a reversibly hygroscopic humectant material such as polyethylene glycol, for example.

The humidity sensors of the present invention have been found to be completely reversible and essentially full range. The improved stability of the sensors of the present invention is especially notable in their ability to exhibit-virtually no calibration drift due to hysteresis or changes in physical stability especially in the higher (about 50 percent) relative humidity ranges. These improvements are accomplished without loss of any desirable properties exhibited by prior sensors such as, for example, a variance in magnitude of resistance versus relative humidity which is well within the range of ordinary measuring equipment, ability to be made extremely small in size and to be compatible with solid state or other circuitry.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates summarily a step-wise process for producing the sensor illustrated in FIG. 2 and both of the figures will be discussed together. The basic, stable substrate upon which the active components of the sensing element are applied is provided in the form of an inert dielectric blank member 10. The inert dielectric blank member may be made of any suitable material which provides a physically rugged substrate, is not I affected by the presence of water vapor, does not chemically react with any of the materials coming in contact therewith (i.e., the material forming the electrode thereon, the material forming the sensitive resistor or the humectant material) and one which allows good adhesion between the electrodes and the substrate and between the resistor material and the substrate. Pure quartz and pure alumina are examples of two materials which have been used successfully as blank members for thesensors of the present invention.

The blank member I may be any size which is con venient for the required application of the sensor. In fact, one of the advantages of this type of sensor is that it can be made extremely small and successful sensors have been made having an overall size as small as approximately 0.45 inches long by 0.2 inches wide by 0.02 inches thick. A pair of electrodes 11 are applied to the blank member to provide the conducting {mechanism between which the resistanceof the humidfity-dependent metal oxide coating is measured. The

material forming the electrodes 11 may be any noble metal conveniently used for such purposes. The one limiting factor in regard to the selection of the electrode material 11 is the temperature used in the sintering step, discussed below. Thus, the melting point of the metal selected for the electrode system must be above the temperature used to sinter the metal oxide coating. Palladium, platinum and rhodium are examples of material which can be successfully used for sintering temperatures up to 1,200C.

The application 12 of the electrodes may be accomplished using any suitable conventional method. The preferred technique, however, appears to be R. F. sputtering in which the desired electrode configuration is determined, the blank member masked exposing the desired configuration and the electrode material sputtered on the blank. As an illustration, the conditions and parameters for one such sputtering process are listed below:

Blank Member The above R. F. sputtering operation was carried out in an R. F. sputtering system, series 2200, Model 2305 from Vacuum Industries of Somerville, Mass. The R. F. sputtering technique appears to impart excellent adhesion between the electrode material and the blank material.

While no particular electrode configuration is required for this sensor of the present invention, an interdigital electrode system, which resembles intermeshed fingers or comb teeth, is preferred because it greatly increases the length of the opposing surfaces of the electrodes.

As discussed above and in the above-mentioned cited reference, certain metal oxides are hygroscopic in nature and exhibit an electrical resistance which changes in value with changes in the relative humidity of the environment to which it is exposed. Those of the elements found in Group VIII, Period 4 of the Periodic Table of the Elements exhibit stable chemical properties and a degree of humidity-related resistance which can be utilized to produce a stable relative humidity sensor. Out of this group the preferred oxides for use in relative humidity sensing elements include those of iron, nickel and cobalt. Thus, successful sensors have been made utilizing nickel oxide (M 0 nickelous oxide (NiO) and iron oxide (Fe O The preferred embodiment of the present invention utilizes the iron oxide (Fe O as the humidity-sensing medium.

In the preferred embodiment, finely divided (normally 1 micron or less in diameter) reagent grade iron oxide (Fe O powder is applied at 13. The preferred method for carrying out this step is to simply apply a layer of the powder approximately 50 microns thick as by sprinkling on the electroded blank. A less preferred but also workable method is to mix the iron oxide powder with a dispersant binder such as polyvinyl alcohol and apply an aqueous solution of this mixture as by spraying to produce a uniform coating overlapping the electrodes.

An important aspect of the present invention is the method of forming or treating the metal oxide humiditydependent resistance coating which imparts the superior qualities to its performance. This is accomplished in the combination compacting (step 13) and heating and holding step 14 which is carried on without applying pressure to the coating on the sensor blank. Normally, the electroded sensor blank coated with the iron oxide powder is placed in a die and mechanically subjected to low pressure compression in the range of from 10 to 150 psi. The pressure is then released and the coated blank is removed from the die with the iron oxide layer somewhat compacted thereon. The coated blank is then heated to a temperature sufficient to cause sintering within a reasonable time. Extensive testing has indicated that the minimum temperature to achieve true sintering within a reasonable time (4 hours or less) is approximately 800C. As the sintering temperature is raised, the time required to achieve the desired sintered matrix in the iron oxide film is considerably reduced. Thus, successfully sintered iron oxide films have been achieved by heating the coated blanks to llC and holding for 15 minutes in an air-tube furnace or by heating to ll25C and holding for 30 minutes in the same type furnace. Tests carried on at approximately 900C indicate that at that temperature minutes to 2 hours is required to achieve a sintered state. I

It is known that a sintered matrix may be achieved at slightly lower temperatures by increasing the mechanical compression applied to the sample in the compacting step. While this allows the material to be success'-. fully sintered, higher pressures tend to reduce the porosity of the sintered iron oxide matrix and, in order to produce a successful sensor, sintered iron oxide matrix i pust be highly porous in nature. Even at the conditions used, some reduction in porosity or average surface area of the iron oxide is noticed; however, tests indicate that the sintered iron oxide has the same sorption properties after sintering as it had prior to sintering and that only a change in surface area has occurred. Thus, the important inner porosity which is required to retain the humectant material which, along with the absorption and desorption of water vapor from the atmosphere, control the conduction between the electrodes, is preserved with a low pressure compacting step.

Following the heating and holding step 14, the blank now containing a sintered iron oxide coating 15 (FIG. 2) overlapping the electrodes is now cooled in step 16 to room temperature. Subsequent to the cooling of the sensor the humectant material which may be defined generally as a chemical moistener that will, because of its chemical structure, attract large quantities of water vapor, is impregnated into the sensor coating, normally by dipping the coated blank into a solution containing the humectant material. Spraying or other conventional methods may also be used. As mentioned in the above-cited patent a choice of the humectant material is important. The humectant material also must be one which is not readily lost to the atmosphere through evaporation, one having water vapor absorption properties which are highly reversible, i.e., having the ability .to readily attract water vapor molecules in a condition of rising humidity and conversely to lose attracted water vapor molecules in a condition ofdecreasing humidity and one which reaches equilibrium quickly. One compound which has been found to combine the above properties well is polyethylene glycol and this compound in the form of Gafanol E-2 00 manufactured by General Aniline and Film Corporation, New York, NY. has been used successfully both in prior sensors and in sensors of the present invention.

As with previous sensors of the same type, the final step in preparing the sensor for its use as a humidity responsive element is the curing step 18. Curing stabilizes the resistance versus relative humidity characteristics of the sensor and is achieved by exposing the sensor to several repeated cycles in which the relative humidity of the environmental atmosphere is raised from approximately percent to 90 percent and lowered again. This step is normally carried out in a closed chamber having an atmosphere in which the relative humidity can be controlled and cycled over a time period, however, any conventional method in which known amounts of dry and saturated air may be combined to produce a desired relative humidity which can be varied can be used.

After the curing step 18 is completed, electrical leads 19 are attached to the electrodes as at 20 in step 21. The leads can be any conventional, compatable electrical leads and the method of attachment may also be in any well known manner depending on the electrode material. The sensor is then calibrated by again exposing it to known relative humidity conditions and noting the resistance of the element over a substantially full range of relative humidity values. Calibration may be accomplished utilizing a conventional A.C. impedance bridge and a readout meter or recording device to record resistance values for various values of relative humidity to which the sensor is exposed. Because the resistance value obtained at a given relative humidity value is dependent upon the frequency of the A.C. power used in the calibration, it should be the same as that which will exist in the circuit of ultimate use for the sensor.

Sensors made in accordance with the present invention utilizing the high temperature sintering step have been found to possess an iron oxide matrix which shows greatly improved physical adhesion to the electrodes and molecular cohesion between the particles of the iron oxide. Improved retention of the humectant material within the porous iron oxide matrix has also been observed. This latter property, it is theorized, results from greater intermolecular attraction between the polyethylene glycol molecules. It has also been discovered that the greatest amount of conduction between the electrodes occurs at a level quite close to the electrodes, i.e., deep within the porous structure of the iron oxide matrix and, therefore, the upper layers of this porous matrix appear to act as a filter and reservoir to keep airborne impurities from reaching the lower levels and affecting that conductance. As with other sensors of the same type, however, a moisture-permeable membrane may be applied over the finished sensor to both aid in retaining humectant and preventing any impurities from reaching the conductive area and affecting the calibration of the sensor.

The embodiments of the invention in which an exclusive property or right is claimed are defined as follows:

1. A moisture sensitive element comprising spaced metal electrodes, a sintered electrically insulating metal oxide coating overlapping said electrodes, said oxide having a characteristic humidity-dependent electrical resistance, and wherein said sintered metal oxide structure is of a porous nature, the pores thereof being impregnated with a reversibly hygroscopic humectant material, and electrical leads attached to said electrodes.

2. The moisture sensitive element of claim 1 including a substantially inert dielectric blank space member upon which said metal electrodes are applied.

3. The moisture sensitive element of claim 2, wherein said metal oxide is one selected from a group consisting of the oxides of Group VIII, Period 4 of the Periodic Table of the Elements.

4. The moisture sensitive element of claim 3 wherein said metal oxide is Fe O 5. The moisture sensor of claim 1 wherein said humectant material is a stable hygroscopic polymer.

6. The moisture sensor of claim 5 wherein said humectant is polyethylene glycol.

7. The moisture sensitive element of claim 6 including a moisture-permeable membrane superimposed over said coating.

8. The moisture sensitive element of claim 7 wherein said moisture-permeable membrane is polyvinyl alcohol sheet. 

1. A MOISTURE SENSITIVE ELEMENT COMPRISING SPACED METAL ELECTRODES, A SINTERED ELECTRICALLY INSULATING METAL OXIDE COATING OVERLAPPING SAID ELECTRODES, SAID OXIDE HAVING A CHARACTERISTIC HUMIDITY-DEPENDENT ELECTRICAL RESISTANCE, AND WHEREIN SAID SINTERED METAL OXIDE STRUCTURE IS OF A POROUS NATURE, THE PORES THEREOF BEING IMPREGNATED WITH A REVERSIBLE HYGROSCOPIC HUMECTANT MATERIAL, AND ELECTRICAL LEADS ATTACHED TO SAID ELECTRODES.
 2. The moisture sensitive element of claim 1 including a substantially inert dielectric blank space member upon which said metal electrodes are applied.
 3. The moisture sensitive element of claim 2, wherein said metal oxide is one selected from a group consisting of the oxides of Group VIII, Period 4 of the Periodic Table of the Elements.
 4. The moisture sensitive element of claim 3 wherein said metal oxide is Fe2O3.
 5. The moisture sensor of claim 1 wherein said humectant material is a stable hygroscopic polymer.
 6. The moisture sensor of claim 5 wherein said humectant is Polyethylene glycol.
 7. The moisture sensitive element of claim 6 including a moisture-permeable membrane superimposed over said coating.
 8. The moisture sensitive element of claim 7 wherein said moisture-permeable membrane is polyvinyl alcohol sheet. 